In general, headphones generate sounds when a driver unit that is disposed in a housing drives a vibration plate according to an audio signal to vibrate air. Here, acoustic characteristics of headphones are known to depend on a structure of a housing. Specifically, acoustic characteristics of headphones can change according to a volume of a space provided in the housing, a size of a vent hole that is formed in the housing and is capable of serving as a passage of air, a size of an opening that is formed on a partition wall of the housing and is capable of serving as a passage of air between the inside and the outside of the housing, and the like. Thus, there are a number of technologies proposed in relation to structures of housings in order to improve acoustic characteristics.
For example, Patent Literature 1 discloses a technology for improving acoustic characteristics by providing a tubular duct unit which spatially connects the inside and the outside of a housing on a rear side of the housing that is the opposite side to the side on which a vibration plate of a driver unit is provided.